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Branton Campbell in the lab

Branton J. Campbell

Professor

Department of Physics & Astronomy

Brigham Young University

Provo, UT 84602, USA

Tel: 801-422-5758, Fax: 801-422-0553

Email: branton_campbell[at]byu.edu

//physics.byu.edu/faculty/campbell/

Research Interests

I apply state-of-the-art x-ray and neutron scattering techniques to study local and long-range structures in a variety of complex solids, including fast-ion conductors, ferroelectric relaxors, high-temperature superconductors, and colossal magnetoresistive manganites, where nanoscale structural features influence macroscopic physical properties. This includes the development of symmetry-mode analysis as a tool for the determination, refinement and interpretation of distorted structures involving lattice strains, atomic displacements, magnetic moments and occupational orderings at both commensurate and incommensurate wavevectors.

Symmetry-Mode Structure Solution

Group theory provides the most natural parameter set for describing a distorted crystal structure, since a relatively small number of parameters normally exhibit non-zero values. These symmetry modes are also the ideal parameter set for determining a complicated structural distortion and determining its symmetry without prior assumptions. This was demonstrated in a combined x-ray/neutron study of monoclinic room-temperature tungsten oxide.

Enhanced stability of charge and orbital order in La_0.78Sr_2.22Mn₂O₇

By B. J. Campbell (et al.)

Abstract:

Not only is zigzag charge and orbital (CO) order shown to be present in the highly doped (x=0.61) bilayered manganite system La2-2xSr1+2xMn2O7, it demonstrates an enhanced stability relative to the x=0.5 composition where the ordered state might be expected to have the lowest entropy. No magnetic-field dependence is observed up to 5.25 T. The CO ordering temperature also increases from 210 K at x=0.5 to 230 K, and the broad hysteretic conversion of the CO-ordered state to the A-type AF state at low temperatures is only partial. Phase reorganization in this hysteretic mixed-phase region is seen to involve an exponential time relaxation above T=50 K, while reorganization ceases below 50 K, apparently due to the thermally activated nature of the underlying dynamics.

Polaronic orbital polarization in a layered colossal magnetoresistive manganite

By B. J. Campbell (et al.)

Abstract: The striking anisotropy observed in Huang scattering distributions near the intense Bragg reflections of La1.2Sr1.8Mn2O7 is shown to be the result of orbitally polarized polarons in the paramagnetic insulating state above T-C. X-ray single-crystal diffuse scattering patterns from this bilayered colossal magnetoresistive manganite are calculated in terms of the polaronic local structure and compared with experimental measurements. At 300 K, the polaronic e(g) electrons occupy "out-of-plane" (i.e., 3z(2)-r(2)) orbitals, leading to MnO6 octahedra that are Jahn-Teller elongated along the c axis, perpendicular to the perovskite layers. Between 300 K and T-C, however, the "orbital polarization" is shown to shift into the a-b plane (i.e., 3x(2)-r(2) and 3y(2)-r(2) orbitals), allowing the formation of nanoscale polaron correlations above T-C.

Glass transition in the polaron dynamics of colossal magnetoresistive manganites

By B. Campbell (et al.)

Abstract: Neutron scattering measurements on a bilayer manganite near optimal doping show that the short-range polaron correlations are completely dynamic at high T , but then freeze upon cooling to a temperature T*approximate to310 K . This glass transition suggests that the paramagnetic/insulating state arises from an inherent orbital frustration that inhibits the formation of a long-range orbital- and charge-ordered state. Upon further cooling into the ferromagnetic-metallic state (T-C=114 K) , where the polarons melt, the diffuse scattering quickly develops into a propagating, transverse optic phonon.

Distinct insulating state below the Curie point in Pr_0.7Ba_0.3MnO₃

By B. J. Campbell (et al.)

Abstract:

Magnetization, resistivity, thermal expansion, thermopower, and neutron powder diffraction data are presented for the perovskite manganite Pr0.7Ba0.3MnO3. A distinct ferromagnetic insulating state, similar to that observed in the La0.88Sr0.12MnO3 system, is observed in the temperature range between the paramagnetic-ferromagnetic transition at T(FM)approximate to180 K and the broad metal-insulator transition near T-p=120 K. The large A-site cation-size mismatch appears to suppress the ferromagnetic double-exchange interactions relative to the competing ferromagnetic superexchange interactions, thereby giving rise to the ferromagnetic insulating state.

Linear framework defects in zeolite mordenite

By B. J. Campbell (et al.)

Abstract: Framework defect correlations are identified in zeolite mordenite that explain both X-ray powder diffraction anomalies and diffuse intensity features in previously reported electron diffraction data. The c/2 displacement of linear "chains" of framework material parallel to the main [001] channel are shown to selectively suppress and broaden the l = 2n + 1 Bragg reflections, thereby linking these framework defects to observed trends in a number of diffracted intensity ratios. At the same time, these defects are shown to produce thin sheets of diffuse scattering that are restricted to the l = 2n + 1 planes of reciprocal space, features that have been reported in electron diffraction studies of mordenite. An empirical relationship between the defect concentration and the framework aluminum content is also established.